This is a national stage of Application No. PCT/AT98/00064, filed Mar. 11, 1998.
1. Field of the Invention
The invention relates to a process for the reduction of ores, especially iron ores, to iron. In this process, the ore at a high temperature is brought into contact with a reduction agent which is preferably a carbon-containing gas.
2. Brief Description of the Related Art
DE 40 30 093 A1 describes a process for the direct reduction of iron ore in a shaft furnace with a reduction gas containing hydrogen and carbon monoxide. Top gas extracted from the shaft furnace is subsequently mixed with a methane-rich gas, and the gas mixture is then converted to reduction gas in a reformer.
U.S. Pat. No. 4,537,626 reveals a process in which the reduction gases exiting a steel converter are heated in a heat exchanger with a carbon-containing material and are subsequently introduced into a metallurgical reduction reactor. This method has the particular disadvantage of not being able to influence the quality of the reduction gas after it exits the converter. An even reduction process in the reduction reactor is not ensured and its product quality is subject to extreme fluctuations as a consequence.
U.S. Pat. No. 4,175,951 reveals a process for the production of a hot reduction gas stream in which a preheated reduction gas stream is mixed with the combustion products of a gaseous hydrocarbon material. Because an external source of fuel must be available for preheating the reduction gas as well as for the heating of the reduction gas to the desired final temperature, this process is judged as disadvantageous due to the continual operational costs.
EP-A 004 1861 describes a heated heat exchanger for heating gases with solid fuels, though in this process the combustion gases are mixed with the gases to be heated. However it is a disadvantage in this process that the gases are mixed.
EP-A 0 663 445 describes a gas-gas, tube-bundle heat exchanger for high temperatures. In this heat exchanger, the tube bundle can be replaced after removing the cap. It is also a disadvantage that the gases are mixed in this system.
WO 94/10512 describes a gas heater in which special emphasis is placed on maintaining the cleanness of the gas. Heating the gases is accomplished by means of a heated perforated impingement plate which is designed to be regulated by temperature.
EP-A 0 056 603 describes a Cowper for blast furnace processes. The Cowper is lined with a ceramic refractory which is characterized by high chemical and thermal strength. This system is disadvantageous insofar as the Cowper possesses a high thermal inertia due to its construction.
DE-C 3 213 204 depicts heat exchangers, preferably for cooling flue gas, with heads and high-temperature-resistant pipes of which the replacement pipes are arranged in a longitudinal direction. The support head is of concrete, and the pipes are positioned in a head which is longitudinally moveable. The heat exchanger is operated in countercurrent flow, and the guide head is moveable to account for the heat expansion of the pipes due to the changes in temperature. The construction is very sturdy; however because the materials contain iron, they lend themselves to corrosion and abrasion.
DE-C 3 142 485 depicts a glass-tube heat exchanger for cooling aggressive and hot flue gases. In this system, the glass tube bundle is positioned perpendicular to the flow of the gas which is fed to the tubes from lateral slits. A cross-section of the entry slits are variable, however it is rather difficult to employ this system in heating the mentioned gases.
The DE-C 3 333 057 has also been recommended as a heat exchanger consisting of glass tubes onto which gases are blown from the side and the housing of which is jacketed. The heating elements are built into the hollow space within the housing wall and minimize the condensation of highly volatile components. This system is not, however, conceived for the heating of gases in the required areas.
In one of the current processes, the reduction gas is brought to the required reaction temperature before it is fed into the reduction container. Currently, this occurs by means of metallic heat exchangers, some of which are available as throw-away heat exchangers. In cases of high CO content, the conventional designs have the problem of carbide build-up in the material due to the dissociation of the CO. These carbides lead to decomposition of the material as soon as the saturation point is surpassed. Even in the event of reduction gases which contain high concentrations of H.sub.2, there is a decomposition of the material as well. In the conventional designs of the various systems, this problem is solved by a partial combustion of the reduction gas. This leads to compromises in the quality of the reduction gas.
This means that the heat exchangers made of iron materials make it nearly impossible to supersede certain temperature zones and to fall below oxidation potentials because the attrition of the heat exchanger would not comply with industrial tolerances. Furthermore, the current heat exchangers represent a safety problem.